Commons Collections

Apache Commons Collections

https://commons.apache.org/proper/commons-collections/
The Java Collections Framework was a major addition in JDK 1.2. It added many powerful data structures that accelerate development of most significant Java applications. Since that time it has become the recognised standard for collection handling in Java.

Commons-Collections seek to build upon the JDK classes by providing new interfaces, implementations and utilities. There are many features, including:

Bag interface for collections that have a number of copies of each object

BidiMap interface for maps that can be looked up from value to key as well and key to value

MapIterator interface to provide simple and quick iteration over maps

Transforming decorators that alter each object as it is added to the collection

Composite collections that make multiple collections look like one

Ordered maps and sets that retain the order elements are added in, including an LRU based map

Reference map that allows keys and/or values to be garbage collected under close control

Many comparator implementations

Many iterator implementations

Adapter classes from array and enumerations to collections

Utilities to test or create typical set-theory properties of collections such as union, intersection, and closure

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